[THIN_HISTORY] Decouple changesets and history from ethdb.Database...

[THIN_HISTORY] Decouple changesets and history from ethdb.Database (mutation/bolt_db/badger_db) (#432)

* Prepare for refactoring

* Decoupling

* Further cleanup of mutation

* Further cleanup of mutation

* Remove some tests (temporarily)

* Fix linter

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Recover mutation_test cases

* Fix lint

* Fix WalkAsOf test

* Fix lint

* Fix TODO, linter

* Fix lint

* Fix lint

* Fix tests for THIN_HISTORY

* Fix lint

* Fix lint

* Reduce visibility of DecodeStorage

* Revert "Reduce visibility of DecodeStorage"

This reverts commit 2e11e16ea095b75a62cdddb77b9477eabc259e67.